1. Field of the Invention
This invention is related to a magnetic head and disk assembly and, in particular, to a head gimbal assembly for use with a magnetic head and disk assembly. More particularly, the invention is related to a head gimbal assembly, and a method for loading the slider of a head gimbal assembly such that, during data transduction, a desired distance is maintained between a read/write rail of the slider and the disk of the magnetic head and disk assembly.
2. Related Art
FIG. 1A is a simplified side view of a magnetic head and disk assembly 100 including a slider 101 positioned over a disk 102 mounted on a spindle 103. FIG. 1B is a simplified plan view of the magnetic head and disk assembly 100 showing additionally a load arm 104 attached to a base plate 105 which is mounted on an actuator arm 106. The slider 101 is mounted on a flexure (not shown for simplicity) which is, in turn, attached to the end of the load arm 104 opposite the end of the load arm 104 attached to the base plate 105. The flexure, load arm 104 and base plate 105 together form a suspension. The suspension and slider together form a head gimbal assembly (HGA). During operation of the magnetic head and disk assembly 100, the actuator arm 106 is driven by a motor (not shown) to move the HGA so that the slider 101 is located over a desired area of the disk 102.
FIG. 1C is a plan view of the slider 101, taken along section A--A in FIG. 1A, showing surfaces of the slider 101 that are adjacent the disk 102. The slider 101 has two rails 101a and 101b. The rail 101b (read/write rail) includes a data transducer that is used to magnetically read and write data from and to the disk 102. The rail 101a (inactive rail) is used to balance the read/write rail 101b; the inactive rail 101a does not perform any data operations.
Each of the rails 101a and 101b has a beveled edge 101c and 101d, respectively. The beveled edges 101c and 101d are located so that the beveled edges 101c and 101d face into the direction of rotation 107 of the disk 100. The beveled edges 101c and 101d help lift the slider 101 off the disk 102 when the disk 102 starts to rotate at the beginning of operation of the magnetic head and disk assembly 100, as explained in more detail below.
When the magnetic head and disk assembly 100 is not operating, the disk 102 is at rest and the rails 101a and 101b of slider 101 are held in contact with the disk 102 by a spring force F ("gram loading") applied by the load arm 104 as shown in FIG. 1A. (Though the spring force F may be applied to an area of the slider 101, the spring force F can be resolved into a single force applied at a point, i.e., the "effective point of application," and is so shown in FIG. 1A.) During operation of the magnetic head and disk assembly 100, the disk 102 is driven to rotate through the spindle 103 by a motor (not shown). This rotation causes an airflow that strikes the beveled edges 101c and 101d, giving rise to aerodynamic forces A.sub.1 and A.sub.2 acting on rails 101a and 101b, respectively, as shown in FIG. 1A. (Though, in reality, the aerodynamic forces A.sub.1 and A.sub.2 act over the entire surface of the rails 101a and 101b, respectively, adjacent the disk 101, these forces can be resolved into a single force applied at a point, i.e., the "effective point of application," and are so shown in FIG. 1A.) As the speed of rotation of the disk 102 increases to operating speed, the aerodynamic forces A.sub.l and A.sub.2 become sufficiently large to cause the rails 101a and 101b, respectively, to rise off of the surface of the disk 102.
Ideally, at the operating speed of the disk 102, the equilibrium between the load arm spring force F and the aerodynamic forces A.sub.l and A.sub.2 results in a desired spacing of the rails 101a and 101b from the disk 102. The read/write rail 101b must be held above the disk 102 within a specified tolerance of the height necessary for proper data transduction as dictated by the performance characteristics (i.e., recording density of the disk 102, rotational speed of the disk 102, characteristics of the data transducer, etc.) of the magnetic head and disk assembly 100. The inactive rail 101a must be held above the disk 102 at a height sufficient to ensure clearance between the inactive rail 101a and the disk 102. Desirably, the inactive rail 101a is at the same height as, or a slightly greater height than, the read/write rail 101b above the disk 102. Illustratively, the height between the read/write rail 101b and the disk 102 is 5 microinches with a tolerance of 1 microinch, and the height of the inactive rail 101a is 0 to 1 microinches higher than the height of the active rail 101b. (The height of the inactive rail 101a may alternatively be specified by designating an allowable range of roll angles of the slider 101. The roll angle of a slider is measured in the plane of FIG. 1A.)
The magnitude of the force A.sub.1 or A.sub.2 is a function of (among other things) the speed of the airflow at the location of the effective point of application of the force A.sub.1 or A.sub.2. The speed of the airflow at any given disk location is proportional to the rotational speed of the disk 102 at that location, which is, in turn, proportional to the distance of that location from the axis of rotation 103a of the spindle 103. Therefore, the magnitude of each of the forces A.sub.1 and A.sub.2 is a function of the distance r.sub.1 and r.sub.2, respectively, between the axis of rotation 103a and the effective point of application of the force A.sub.1 or A.sub.2. As the distance from the axis of rotation 103a to the effective point of application of the aerodynamic force, e.g., force A.sub.1, increases, the magnitude of the aerodynamic force, e.g., force A.sub.1, increases.
Since the effective point of application of the force A.sub.2 acting on the read/write rail 101b is a greater distance r.sub.2 from the axis of rotation 103a than the distance r.sub.1 of the effective point of application of the force A.sub.1 acting on the inactive rail 101a, the force A.sub.2 is greater than the force A.sub.1. Therefore, assuming that the effective point of application of the spring force F is midway between the effective points of application of the aerodynamic forces A.sub.1 and A.sub.2, during operation of the magnetic head and disk assembly 100, the read/write rail 101b will be forced further away from the disk 102 than the inactive rail 101a, i.e., the slider 101 will have a non-zero roll angle. In practice, if the roll angle of the slider 101 is sufficiently large, then the desired heights of the rails 101a and 101b (as described above) may not be maintained.
As noted above, it is desirable that the read/write rail 101b be at a particular height above the disk 102 and that the inactive rail 101a be at a sufficient height above the disk 102. This may be done by applying a gram loading (i.e., spring force F) of appropriate magnitude to the slider 101 in a manner that gives rise to a differential load on the rails 101a and 101b that compensates for the differential aerodynamic forces A.sub.1 and A.sub.2 acting on the rails 101a and 101b, respectively, so as to keep the rails 101a and 101b at the desired heights. The differential loading can be accomplished by, for example, adjusting the effective point of application of the spring force F. If the effective point of application of the spring force F is moved toward the effective point of application of the aerodynamic force A.sub.2, the height of rail 101b above the disk 102 will decrease and the height of rail 101a above the disk 102 will increase. Likewise, moving the effective point of application of the spring force F toward the effective point of application of the aerodynamic force A.sub.1, results in decreasing the height of rail 101a above the disk 102 and increasing the height of the rail 101b. By appropriately locating the effective point of application of the spring force F, the heights of rails 101a and 101b above the disk 102 can be controlled as desired.
As illustrated by FIGS. 2A and 2B, the effective point of application of the spring force F is moved by incorporating a "bonded offset" 203 (FIG. 2B) into the assembly of the slider 101 to the load arm 104. In FIG. 2A, the slider 101 is assembled to the load arm 104 such that the effective point of application of the spring force F lies in a plane 201 that is coincident with a plane 202 that lies midway between the effective points of application (not shown) of the aerodynamic forces A.sub.1 and A.sub.2. (Note that both planes 201 and 202 are parallel to the longitudinal axis of symmetry of the slider 101.) Since the spring force F is applied midway between the aerodynamic forces A.sub.1 and A.sub.2, the spring force F does not result in a moment that affects the roll angle of the slider 101, i.e., the spring force F equally affects the heights of the rails 101a and 101b.
In FIG. 2B, the slider 101 is assembled to the load arm 104 such that the plane 201 is offset (bonded offset 203) from the plane 202. In FIG. 2B, the bonded offset 203 is in the direction of the rail 101b, i.e., the effective point of application of the spring force F in FIG. 2B is moved toward the rail 101b relative to the effective point of application of the spring force F in FIG. 2A. Consequently, the rail 101b is at a lower height, and the rail 101a is at a higher height, than the corresponding heights when the slider 101 is assembled to the load arm 104 as in FIG. 2A. For example, with proper choice of bonded offset 203, the equilibrium between spring force F and aerodynamic forces A.sub.l and A.sub.2 results in rails 101a and 101b being at equal heights from the disk 102. Conversely, the slider 101 can be assembled to the load arm 104 so that the plane 201 is offset in the opposite direction so that the rail 101b is at a higher height and the rail 101a is at a lower height than the corresponding heights in FIG. 2A.
There are several problems with this approach to loading the slider 101 that cause the rails 101a and 101b to be at other than the desired heights above the disk 102. First, in practice, the gram loading (i.e., spring force F) cannot be controlled precisely, i.e., a tolerance is associated with the gram loading. Typically, a nominal gram loading of 9.5 grams is used to bias the slider 101 against the disk 102. A typical tolerance is 0.75 grams. Thus, a magnetic head and disk assembly 100 assembled to achieve the desired height of the rails 101a and 101b above the disk 102 for a gram loading of 9.5 grams may, in fact, be subject to a gram loading of as little as 8.75 grams or as much as 10.25 grams.
FIG. 3A illustrates the effects of gram loading tolerance. For actual gram loading that is less than the nominal value, the slider 101 will be moved up (slider 301a represented by the dashed-dotted line) relative to the desired position. For actual gram loading that is greater than the nominal value, the slider 101 will be moved down (slider 301b represented by the dashed line) relative to the desired position. Thus, as a result of gram loading tolerance, rail 101a and/or rail 101b may be at a height, e.g., height 302a or 302b, from the disk 102 that is unacceptably higher or unacceptably lower than the desired height 302.
Second, in bonding the slider 101 to the load arm 104, the slider 101 may be misaligned so that the desired bonded offset 203 is not obtained. In practice, alignment of the slider 101 with the load arm 104 has, at best, a tolerance of about 0.002 inches. As a result of this misalignment, the plane 201 containing the effective point of application of the spring force F will be shifted from the desired location. The resulting equilibrium between spring force F and aerodynamic forces A.sub.1 and A.sub.2 results in rails 101a and 101b being at different heights from the disk 102 than desired, the lowered rail being the one toward which the spring force F is offset from the desired bonded offset 203.
FIG. 3B illustrates the effects of alignment tolerance in positioning of the slider 101 with respect to the load arm 104. If the slider 101 is misaligned toward the rail 101b, the rail 101b is tilted toward the disk 102 (as shown by the slider 301c). If the slider 101 is misaligned toward the rail 101a, the rail 101a is tilted toward the disk 102 (as shown by the slider 301d). Thus, as a result of alignment tolerance, the height of rail 101a and/or rail 101b may be unacceptable.
Third, the slider 101 may have a rotary spring bias (i.e., static attitude bias) with respect to the disk 102 when the disk 102 is at rest, such that one or the other of the rails 101a or 101b is closer to the disk 102 than the other (i.e., non-zero roll angle of the slider 101). This static attitude bias may arise from, for instance, the misorientation of the load arm/base plate attachment, load arm/flexure attachment or flexure/slider attachment; twist in the load arm or flexure; or manufacturing variations in formation of the load arm 104, flexure or slider 101. Static attitude bias can result in one or both of the rails 101a and 101b flying at an unacceptable height above disk 102. The effects of static attitude bias are also illustrated in FIG. 3B by the sliders 301c and 301d.
One method that has been used in an attempt to address the above problems is to, during assembly, vary the position of the slider 101 with respect to the load arm 104 to obtain a desired differential load between the gram loading on the read/write rail 101b and the gram loading on the inactive rail 101a. Typically, in this method, the gram loading of the read/write rail 101b is made a predetermined amount larger than the gram loading of the inactive rail 101a. If the overall gram loading of the slider 101 had no tolerance associated with it, this method could be used to produce the desired spacing between the rails 101a and 101b and the disk 102, since the effects of alignment tolerance of the slider 101 relative to the load arm 104 and static attitude bias of the slider 101 are eliminated.
However, as noted above, the overall gram loading is only known within a certain tolerance. Thus, the exact differential load necessary to provide the desired spacing of the rails 101a and 101b from the disk 102 cannot be known.
Thus, there is a need for a head gimbal assembly and a method of loading a slider for use with a magnetic head and disk assembly such that the read/write rail of the slider is maintained at a desired height above the disk, the roll angle of the slider is controlled, and clearance is maintained between the inactive rail and the disk.